1. Field of the Invention
This invention relates generally to the field of Eddy current proximity sensors, and more particularly to an Eddy current position sensor having a progressively wound ferromagnetic core to produce a conical shaped electromagnetic field and a linear voltage output.
2. Prior Art
Eddy current or inductive proximity sensors are well known. These sensors typically include a length of conductive wire wound in coil fashion around a non-ferrous core of either air, plastic or ceramic or a ferrite core. The wire coil is electrically energized by a low level radio frequency current to produce an electromagnetic field surrounding the coil/core assembly. This electromagnetic field may be measured by a parameter called "Q" which is a function of the excitation frequency, the inductance of the coil and the resistive losses of the coil.
When a metallic material forming a target is placed within this magnetic field, Eddy currents will be generated on the surface of the target. These Eddy currents absorb energy from the oscillating coil, increasing the energy loss of the coil and reducing the Q of the excited field. Reduction of Q is proportional to the proximity of the metallic target to the coil. Various means have been utilized to accurately and linearly measure the Q of the electromagnetic field and thus the distance between the core and the magnetic target. One such device is disclosed in U.S. Pat. No. 3,609,580 to Thompson, et al. which teaches such an electrical sensing apparatus having an inductive coil and a uniquely configured oscillatory circuit for both exciting and measuring the response of the electromagnetic field produced by proximity of a metallic object to the core.
An automatic liquid level sensor utilizing Eddy current principles is disclosed in U.S. Pat. No. 4,497,205 invented by Zuleuf, et al. This disclosure teaches a magnetic float moving vertically in response to variations in liquid level with respect to an alternating current driven coil. The coil which is located at one end of the float path is driven by a pulse generator thus producing a variable part of the core of the coil. Thus, the device claims to be useable to provide a signal at a predetermined liquid level or an output proportional to the level of the liquid.
Applicant is also aware of two other more distantly related devices, one disclosed in U.S. Pat. No. 4,080,828 invented by Akita which teaches a liquid level detecting apparatus which detects incremental step movements as measured by frequency changes caused by a ring float. The output is not proportional to the positioning of the ring float, however. Clark in U.S. Pat. No. 3,140,608 teaches a liquid level gauge in the form of a capacitance probe using the capacitance of a wire coil on a TEFLON core. Eddy current principles are likewise not employed in this invention.
The concept of linear oscillators is also well-known for use in conjunction with Eddy current proximity sensors. Such circuits are used to provide the radio frequency exciting current to the coil assembly and to provide a measurement of the circuit "Q". In such an arrangement, a constant current is applied to a sensor coil to form a resonant oscillator circuit whose frequency is a function of the coil inductance and the capacitance of a capacitor parallel wired with the coil. The natural frequency of such an oscillator is determined by well-known formula based upon both the inductance of the coil and the capacitance value. Feedback from this oscillator arrangement is provided to a current switch driver which allows the current to the coil to be switched in sync with the oscillator frequency to sustain oscillation. The voltage produced from such an oscillator circuit is a function of the current level passing into the coil, the coil inductance and the "Q" of the coil. As the sensor coil/core approaches a metallic target, the "Q" is reduced.
However, none of the prior art devices above described or otherwise known to applicant produce a linear output signal which is proportional to the axial positioning of a ringed magnetic target surrounding the coil/core. Most of these prior art devices are used as proximity sensors which produce a responsive signal at a predetermined voltage output level. Otherwise, the voltage output is non-linear at best, thus producing irregular and unpredictable target position responses.
The present invention provides a uniquely configured progressively spaced coil wound about a ferromagnetic core which, in combination with a linear oscillator, produces a virtually linear voltage output in response to movement of a magnetic ring positioned about a conical shaped electromagnetic field produced by the elongated coil/core arrangement.